(1) Field of the Invention
This invention relates to semiconductor integrated circuit devices, and more particularly to a method for making field effect transistors (FETs) with different gate oxide thicknesses for various integrated circuit applications, such as forming a thicker tunnel oxide for non-volatile (flash) memory while providing a thinner gate oxide for high-performance logic circuits. The method uses a patterned thin silicon nitride (Si3N4) layer and a single oxidation step to provide a good quality thicker FET tunnel gate oxide while concurrently forming a good quality thinner FET gate oxide for the high-speed CMOS logic.
(2) Description of the Prior Art
Merged semiconductor logic and memory circuits are finding extensive use in the electronics industry for merged logic/memory devices. These deep submicrometer circuits require different gate oxide thicknesses to optimize the FET performance. Typically flash (non-volatile) memory and peripheral input-output (I/O) circuits require a thicker gate oxide, while CMOS logic circuits require a thinner gate oxide for increased performance (switching speed). For example, flash memory and peripheral circuits require a thicker oxide of 50 Angstroms or more and operate at a gate voltage of about 3.0 to 5.0 volts, while CMOS logic devices have effective gate oxides that are less than 25 Angstroms and operate at lower gate voltages (V) of between about 1.8 V and 2.5 V to achieve higher switching speeds.
One prior art method of achieving dual-thickness gate oxides for FETs is depicted in the schematic cross-sectional views of FIGS. 1 and 2. A field oxide 12 is formed in and on the silicon substrate 10 using conventional means to surround and electrically isolate the logic device areas L, as shown in the right potion of FIG. 1, and as shown in the left portion of FIG. 1 for the memory and peripheral device areas M. A first gate oxide 13 is grown on both the logic and memory/peripheral device areas. Next, as shown in FIG. 2, a photoresist mask 16 is used to mask the gate oxide 13 over the memory device areas M, and then a wet etch is used to remove the gate oxide 13 over the logic device areas L. The photoresist mask 16 is then removed by plasma ashing and/or by stripping in a photoresist stripper. Unfortunately, the photoresist 16 in direct contact with the thin gate oxide 13 on the silicon substrate 10 surface results in contamination that causes reliability problems. Next as shown in FIG. 3, prior to growing a thinner second gate oxide 15 on the logic device areas L, it is necessary to remove the native oxide in the logic device areas using a wet etch. However, the exposure of the first gate oxide 13 to the wet etch makes it difficult to accurately control the thickness of the thicker gate oxide, composed of oxide layers 13 and 15, over the memory device areas when the second oxidation step is performed.
Numerous methods of forming logic circuits with embedded memory devices having dual gate oxides have been reported in the literature. One method of making a dual-thickness gate oxide is described in U.S. Pat. No. 5,960,289 to Tsui et al. In Tsui""s method a first gate oxide is deposited and a protection layer (Si3N4) is deposited. The two layers are then patterned to expose some of the device areas. The exposed device areas are oxidized to form a thin second gate oxide for high-performance circuits, while the first gate oxide and the protection layer serve as the thicker gate dielectric for the DRAM and the SRAM cells.
Sugaya in U.S. Pat. No. 4,945,068, teaches a method for forming a thick and thin dual-thickness gate oxide using only a single oxidation step. The method achieves the dual thickness by using an ion implant mask and implanting (injecting) nitrogen (N2) ions into some of the silicon substrate device areas while masking from implant other device areas. A thermal oxidation is performed in which the oxide grows slower on the N2 implanted areas than in non-implanted device areas.
Ong in U.S. Pat. No. 5,880,041 describes a method for using high-pressure oxidation to grow a gate oxide on a silicon substrate. The high pressure increases the oxidation rate (Angstroms/minute) and decreases the oxidation time, which reduces the thermal budget.
Fang et al. in U.S. Pat. No. 5,668,035 describe a method for forming a dual gate oxide for memory with embedded logic technology. The method grows a first gate oxide over memory and logic device areas. Then a polysilicon layer is deposited and patterned to expose the first gate oxide over the logic device areas and to protect the first gate oxide over the memory device areas. The first gate oxide is removed by wet etch over the logic device areas and a thinner second gate oxide is grown over the logic device areas.
Matsuoka et al. in U.S. Pat. No. 5,926,741 describe a method for forming a gate dielectric layer without generation of a natural oxide film. The method involves cleaning the silicon wafer in an inert gas ambient and replacing the ambient gas with an oxidizing gas and applying heat to form the gate oxide on the wafer.
However, there is still a need in the semiconductor industry for fabricating dual-thickness gate oxides using a simple cost-effective process, while preventing native oxides from forming on device areas, as well as avoiding boron penetration in the thin gate oxides for P-channel FETs.
It is a principal object of the present invention to form dual gate oxides on a single integrated circuit chip for field effect transistors (FETs) for making a thin effective gate oxide for high-performance logic circuits and a thicker gate oxide for non-volatile memory and/or peripheral circuits.
It is another object of the invention to use a patterned thin silicon nitride (Si3N4) layer to form the thin gate oxide while concurrently forming the thicker gate oxide using a single oxidation step.
A further objective of this invention is to use a single oxidation step to reduce processing complexity and to improve product reliability.
The method for making this dual gate oxide using a patterned thin Si3N4 layer for CMOS logic circuits and for non-volatile memory is now briefly described. The method begins by providing a semiconductor substrate. The substrate is typically a single-crystal silicon having a  less than 100 greater than  crystallographic orientation. A field oxide is formed surrounding and electrically isolating CMOS logic device areas and memory device areas. Typically for these more advanced high-density integrated circuits, the field oxide is shallow trench isolation as commonly practiced in the industry. By the method of this invention the dual gate oxides are now formed by removing any native oxide that has inadvertently formed on the logic and memory device areas. Next a very thin blanket Si3N4 layer (e.g., 10-20 Angstroms) is deposited on the substrate using chemical vapor deposition (CVD). Preferably the native oxide is removed in situ just prior to depositing the Si3N4 layer to avoid reforming a native oxide. The Si3N4 layer is then patterned using a photoresist mask and a hot phosphoric acid (H3PO4) etch. The Si3N4 is patterned to leave portions over the logic device areas while exposing the surface of the memory and/or peripheral device areas. A key feature of this invention is that the photoresist mask used to pattern the Si3N4 does not come in direct contact with a gate oxide, as in the prior art. Therefore the gate oxide and/or the substrate are not contaminated by the photoresist. Any second native oxide that has inadvertently formed on the memory device areas is now removed in a dilute hydrofluoric (HF) acid solution or an HF vapor etch, while essentially leaving unetched the patterned Si3N4 over the logic device areas. The surface of the substrate is then subjected to a single oxidation step to form a thicker gate oxide (greater than or equal to 50 Angstroms) on the memory device areas, while concurrently converting the thin patterned Si3N4 layer to a nitrogen-rich SiO2 layer having an effective oxide thickness of less than or equal to about 25 Angstroms over the logic device areas. The oxidation is carried out for a time sufficient to convert the Si3N4 to a nitrogen-rich SiO2 and to partially oxidize the underlying surface of the silicon substrate to form a good interface. After completing this improved dual gate oxide, conventional processing is used to form FET devices. For example, the FET gate electrodes can be formed next by depositing and patterning a doped polysilicon layer or a polycide layer.